A manuscript on tumor growth curves is being revised. A new second order non-linear differential equation for monotonic growth is proposed. This equation contains all classic models such as the Gompertz, logistic and exponential as special cases, and introduces an infinity of intermediate curves with varying inflection points. The solutions to the growth equation are obtained in closed form, and can be expressed in a time-free form which allows inter-comparison of the shape of different growth curves independent of age and dependent only on tumor extent. Numerical solution of equations to fit the general growth curve to data is unique. This equation should facilitate comparing data from different laboratories and provide an intrinsic characterization of the growth phenomena.